*{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}
.box{          
    width: 250px;
    height: 250px;
    position: relative;
    margin: 50px auto;
}
.small{
    width: 250px;
    height: 250px;
    border: 1px solid black;
}
.small>img{
    display: block;
    width: 100%;
    height: 100%;
    cursor: pointer;
}
.mask{
    width: 80px;
    height: 80px;
    background-color: rgba(236, 58, 58, 0.452);
    position: absolute;      
    top: 0;
    left: 0;
    display: none;
  }
.big{
    width: 250px;
    height:250px;
    border: 1px solid rgb(156, 193, 236);
    position: absolute;
    top: 0;
    left:250px;
    display: none;
    overflow: hidden;
}
.big>.img{
    width: 800px;
    height: 800px;
}
.big>.img>img{
    display: block;
    width: 100%;
    height: 100%;
}